Doc: Remove external libraries from documentation

This patch removes all documentes listed under the
path '/lib/ext' from the Sphynx build TOC and adds
the patch to exclusion list.

Change-Id: I7852b030b946d68b359c31ce4a785920a07f4ff5
Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>